1. Field of the Invention
The present embodiments relate to an improved method of synthesizing high molecular weight polyquaternium-1 and related molecules for use as antimicrobial agents in contact lens solutions.
2. Description of the Related Art
Quaternary ammonium polymers in which the ammonium moieties are part of the linear polymeric chains have been used as antimicrobial agents in several industries. Polyquaternium-1 (PQ1) is a polymeric quaternary ammonium anti-microbial agent that has been used, for example, in preserving ophthalmic compositions and disinfecting contact lenses. PQ1 is effective against bacteria, algae and fungi. Its chemical name is poly[(dimethyliminio)-2-butene-1,4-diyl chloride], α-[4-[tris(2-hydroxyethyl) ammonio]-2-butenyl]-ω-[tris(2-hydroxyethyl)ammonio]-dichloride.
U.S. Pat. No. 3,931,319, which is hereby incorporated in its entirety by reference, describes a two-step method for PQ1 synthesis which requires a high reaction temperature. This leads to significant degradation of the target molecule into impurities from which the desired PQ1 is difficult to separate.
U.S. Pat. No. 4,027,020, which is hereby incorporated in its entirety by reference, describes a procedure for polyquaternium-1 synthesis which results in less degradation of the resulting PQ1 than the method described in U.S. Pat. No. 3,931,319 but still produces a rather low yield. The procedure disclosed in U.S. Pat. No. 4,027,020 entails mixing 1,4-bis-dimethylamino-2-butene with triethanolamine (TEA), the molar ratio of the 1,4-bis-dimethylamino-2-butene to the TEA amine being from 2:1 to 30:1 followed by the addition of 1,4-dichloro-butene to the mixture in a molar amount equal to the sum of the molar amount of the 1,4-bis-dimethylamino-2-butene plus one-half the molar amount of TEA. The reaction time is 1-10 hours.
A major weakness of the method taught in U.S. Pat. No. 4,027,020 is that the TEA end-capping efficiency is low. As such, the final product contains a significant amount of polymers with no end caps or polymers end-capped with groups other than TEA. These malformed polymers are difficult to separate from polyquaternium-1 because of the similarity in the main chain of the polymeric molecules. Degraded or malformed polymers of PQ1 have reduced anti-bacterial efficacy and cannot substitute for PQ1 in clinical use.
Soft contact lenses usually attract and accumulate quaternary ammonium antimicrobial agents during the lens cleaning/disinfecting/storing cycles. The accumulated antimicrobial agents in the lens are subsequence released once the lens is put in to the eye, causing the contact lens wearer's eye irritation. An effective way to reduce the antimicrobial agents lens uptake is to use low concentration antimicrobial agents. This requires that the antimicrobial agents are of high efficacy. Another way to reduce the eye irritation is to use less cytotoxic antimicrobial agents in MPS.